Our invention relates to spring clutches, and more particularly, to spring clutches in which the force controlling the output element results from contact with an end, or tang, of the clutch spring. As the output element of the clutch rotates, the direction of this force also rotates. In some applications of the clutch, the combination of this rotating force with other more constant forces produces an undesirable surging, or variation in the force required to operate the blind. Also, in order to balance the moment produced by this force, frictional forces are internally generated within the clutch which make its operation more difficult.
U.S. Pat. Nos. 4,372,432 and 4,433,765 both disclose spring clutches having the disadvantage described above. These clutches are intended principally for raising and lowering window shades, venetian blinds, pleated shades and other window treatments that move vertically. These devices are inexpensively built, manually operated devices, without ball or roller bearings of any sort, in which coaxial plastic parts support the weight and ride on one another. High operating force, frictional drag or variations in the force required to move the blind are perceived to be unpleasant, and give an impression of rough operation and poor quality. Nevertheless, because of the construction, frictional drag and uneven operating force are intrinsically present in these prior art devices. Our invention provides a means for minimizing the frictional drag and the variations in operating force.